Technology for automatic microwave oven cooking has generally been developed with an eye towards operational simplification and enhancement of consumer convenience. Various types of sensors have been used in prior art microwave ovens which have automatic cooking functions.
For example, prior art microwave ovens have used sensors located in the heating chamber for detecting temperature, sensors for detecting humidity, sensors for detecting gases generated during cooking, sensors for detecting vapor, and sensors for detecting the weight of food. There are many known methods for controlling cooking by utilizing the output signal of these types of sensors.
Even though many prior art sensors have been used for controlling various types of cooking, none of these methods has been adequate for controlling cooking with weak heat. When cooking with weak heat, which is commonly done, for example, when warming up food or thawing meat or fish, control should be based on the quantity of moisture generated throughout entire cooking period, i.e., humidity. Since it is difficult to detect humidity during warming up or thawing, controlling these types of cooking has been problematic.
Furthermore, when using a single container to successively thaw multiple pieces of meat or fish, prior art ovens that employ a humidity sensor often malfunction due to vapor being generated by the premature boiling of residual water that is left over from the previous thawing operation. As these vapors are not indicative of the current thawing process, they falsely indicate the status of the current thawing process. In order to reduce the frequency of this problem, manufacturers have attempted to explain in owners' manuals the necessity of fully cleaning and drying a container before using that container to thaw a new piece of food. Such efforts, however, may have the undesirable side effect of making microwave thawing cumbersome and inconvenient.
There are many prior art control algorithms which use the foregoing types of sensors. A cooking period control algorithm is one such prior art method which uses humidity sensors or gas sensors. FIG. 1 shows the typical output voltage over time of such gas sensors or humidity sensors during cooking. There is a sharp increase in the output of the sensors when water in the food starts to boil. This is due to generation of vapor or gas as the food is cooked.
One approach to controlling cooking in a microwave oven is to calculate the total amount of heat required to heat the food in the oven. The oven is then operated until this amount of heat has been generated and applied to the food. The total heat can be expressed in following equation: EQU Q=M.times.C.times.(t.sub.f -t.sub.i)+(M.times.B) equation 1,
where Q is the total heat required to heat the food in a microwave oven to an appropriate state, C is the specific heat of the food, M is the quantity of food, t.sub.f is the boiling temperature of moisture in the food, t.sub.i is initial temperature of the food, and B is heat proportional to latent heat and degradation of food.
Because the total heat Q will be the same as the total heat generated by the microwave oven, the total heat Q can also be expressed as follows: EQU Q=T.times.P equation 2,
where T is the total time period of cooking and P is the power output of the microwave oven.
Therefore, the following equation can be obtained by combining equations 1 and 2: EQU T=(M.times.C.times.(t.sub.f -t.sub.i))/P+(M.times.B)/P equation 3.
As the first term of equation 1 represents the period of time from the start of cooking through the boiling of moisture in the food, and the second term represents the period of time from the start of vaporization of the moisture through the completion of cooking, the total period of cooking T can be expressed as follows; EQU T=T.sub.1 +K.times.T.sub.1 equation 4,
wherein, EQU T.sub.1 =(M.times.C.times.(t.sub.f -t.sub.i))/P, EQU K=B/(C.times.(t.sub.f -t.sub.i)),
and
K is a cooking constant which depends on the kind of cooking that is desired.
Thus, by using an appropriate cooking constant K, automatic cooking control can be achieved by an operator simply pressing control buttons on the microwave oven to indicate the desired type of cooking. This is so because T.sub.1 can be readily determined, as follows. A reference detection point is set based on when the food starts to boil, namely when the output signal of the sensor rises sharply. The reference detection period T.sub.1 will then be the time period from the start of cooking until the output of the sensor reaches reference detection point.
According to equation 4, the microwave oven should be operated for a first time period T.sub.1 and then for an additional time period equivalent to K.times.T.sub.1, i.e., the length of time obtained by multiplying the time period T.sub.1 by the cooking constant K. Thus, the total operational time period of the microwave oven will be the reference detection period T.sub.1 added to the product of the reference detection period T.sub.1 multiplied by the cooking constant K. The foregoing is an example of a prior art method used to control general microwave cooking by incorporating prior art humidity sensors, temperature sensors and gas sensors.
As cooking frozen food in microwave ovens becomes more frequent in modern society, the importance of the thawing function in microwave ovens increases. In situations where cooking should be completed prior to water boiling, for instance warming up or thawing food, the aforementioned automatic cooking control method, which relies on the boiling point of water to determine the reference detection point and the period T.sub.1, cannot be used because water should not boil during this type of heating.